This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-029493, filed Feb. 8, 1999, No. 11-066088, filed Mar. 12, 1999; and No. 2000-016482, filed Jan. 26, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a flat display device such as a liquid crystal display device.
For example, a reflection type liquid display device as a conventional flat display device displays an image using external light, so illumination light is short and results in a dark display screen depending on use environments. In particular, this display device cannot be used in a dark place.
There has been developed a semitransmission type liquid crystal display device using a semitransparent reflecting plate (half mirror) for reflecting external light and a backlight arranged on the back surface side of the semitransparent reflecting plate, so as to use the display device as a transmission liquid crystal display device in a dark environment. The utilization efficiency of incident light of the semitransparent reflecting plate is 50% at maximum. The brightness of the display screen is much lower than that of the transmission or reflection type liquid crystal display device.
To overcome this drawback, there has been examined a semitransmission type liquid crystal display device in which a pinhole corresponding to each pixel is formed in a reflecting plate, and a microlens corresponding to each pinhole is disposed. In this liquid crystal display device, when external light is used, the external light reflected by an area except the pinholes of the reflecting plate is used as a light source. When a backlight is used, light transmitted through the pinholes is focused by the microlenses, and the focused light is used to improve the light utilization efficiency.
Even in such a liquid crystal display device, however, light loss caused by the pinholes occurs in using the external light. The transmission type liquid crystal display device using a backlight is frequently used to increase power consumption.
The reflecting plate having pinholes has a complicated structure and must be attached as an independent member to the liquid crystal panel. This produces disparity and greatly degrades the display performance.
There has also been examined a so-called front light type display device in which a light guide plate is disposed on the observation surface side of a reflection type liquid crystal display device, and a linear light source is disposed on the side surface of the light guide plate. However, light is frequently reflected on the front light surface, and display quality such as contrast of the liquid crystal display device greatly degrades.
In the above semitransmission type liquid crystal display device can perform color display by forming a color filter layer. More specifically, the conventional semitransparent type color liquid crystal display device is constructed by stacking a polarization plate, front-surface substrate, color filter layer, drive electrodes, liquid crystal layer, back-surface substrate, semireflecting plate, and back-surface light source in this order. The color filter layer is formed on the front side of the semireflecting plate, i.e., on the observer side.
When the liquid crystal display device functions as a reflection type, external light entering from the front-surface substrate side passes through the color filter layer and liquid crystal layer, is reflected by the reflecting plate, and passes through the liquid crystal layer and color filter layer again, and emerges outside. That is, the external light passes the color filter layer twice along the forward and return optical paths. On the other hand, when the liquid crystal display device functions as a reflection type, light emitted by the back-surface light source passes through the color filter layer only once.
When the display device as the transmission type uses a color filter layer to obtain a sufficient saturation, external light is greatly absorbed by the color filter layer in the display device functioning as the reflection type because the external light passes through the color filter twice. As a result, reflection brightness lowers. Assume that the density of the color filter layer is reduced to obtain sufficient brightness as the reflection type. That is, assume that the wavelength dispersion characteristic of the transmittance of the color filter layer is designed to obtain a desired saturation upon transmission of external light through the color filter layer twice. For example, assume that the Y value of the average transmittance is set to 40% or more. In this case, light emitted by the back-surface light source in the liquid crystal display device functioning as the transmission type passes through the color filter only once, and the saturation becomes short.
As described above, in the conventional semitransmission type color liquid crystal display device, when it functions as the reflection type, the display brightness greatly degrades; when it functions as the transmission type, the display type color density greatly lowers. The conventional semitransmission type color liquid crystal display obtains either optical characteristics.
The present invention has been made in consideration of the above situation, and has as its object to provide a semitransmission type flat display device which can solve the conventional problems and greatly improve light utilization efficiency.
It is another object of the present invention to provide a flat display device capable of displaying an image with sufficient brightness at a sufficient color density even if the display device functions as the transmission or reflection type.
In order to achieve the above object, according to the present invention, there is provided a flat display device comprising a first polarization plate having a polarization axis and adapted to transmit linearly polarized light along the polarization axis, an optical modulation layer located behind the first polarization plate to modulate incident light in accordance with an applied voltage, a selective reflecting layer located behind the optical modulation layer to selectively reflect a first circularly polarized light component of the incident light, and a backlight located behind the selective reflecting layer to output light having intensity peaks in a plurality of predetermined wavelengths,
wherein the selective reflecting layer substantially transmits, of the first circularly polarized light component, light components having wavelengths in a plurality of small regions including the respective predetermined wavelengths, and substantially reflects a light component in regions between the plurality of small regions.
According to the flat display device having the above arrangement, about 95% of the first circularly polarized light components output from a back-surface light source passes through the selective reflecting layer. The selective reflecting layer reflects about 90% of the first circularly polarized light component of the external light. Regardless of whether the flat display device operates as either a reflection type device or the transmission type device, an image can be displayed at a high color purity and high brightness.
Another liquid crystal display device according to the present invention comprises: an optical modulation layer which is sandwiched between a pair of observation-side and back-surface-side transparent substrates opposing each other and has a plurality of liquid crystal pixels arranged in a matrix, the optical modulation layer being adapted to modulate incident light in accordance with an applied voltage; a selective reflecting layer having a plurality of selective reflecting filters arrayed in a predetermined cycle, respectively, on back-surface sides of the liquid crystal pixels to selectively and partially reflect light components having different wavelength bands; and a color filter layer having a plurality of color filters disposed to oppose front-surface sides of the selective reflecting filters and arranged such that a peak wavelength of spectral characteristics in a visible light range of each of the color filters falls within a reflection wavelength band of a corresponding one of the selective reflecting filters.
According to the liquid crystal display device, the plurality of selective reflecting filters for selectively and partially reflecting light components having different wavelength bands in the selective reflecting layer are arrayed in a predetermined cycle so as to correspond to the liquid crystal pixels. Each of the selective reflecting filters reflects a light component of the corresponding selective reflection wavelength band at a predetermined ratio and transmits the rest. Of the external light incident from the observation side, light in a specific wavelength region is reflected in accordance with the reflectance in the selective reflection wavelength band. Of the light of the external light source, which is incident from the observation side, light in a specific wavelength region is transmitted in accordance with the transmittance in the selective reflection wavelength band. Therefore, both external light and light from the light source can be used as source light beams, and semitransmission display is allowed.
The color filter layer is made up of the plurality of color filters having different spectral characteristics. Each color filter is combined with a selective reflection filter having corresponding selective reflection characteristics. More specifically, the peak wavelength of the spectral characteristics in the visible light range of each color filter is designed to fall within the selective reflection wavelength band of the corresponding selective reflecting filter. In accordance with the spectral characteristics, each color filter transmits or absorbs light reflected or transmitted by the corresponding selective reflection filter and output to the observation side. The color filter layer is formed on the observation side closer than the selective reflecting layer and preferably formed on the inner surface of the observation-side transparent substrate.
As described above, each liquid crystal pixel is combined with the selective reflecting filter and color filter having a specific relationship to construct a color pixel of R (red), G (green), or B (blue). In general, since pixels of each color form a stripe, the corresponding reflecting filter and color filter are formed in stripes.
In the flat liquid crystal display device of the present invention, the selective wavelength bands of selective reflecting filters correspond to color pixels, respectively. Such a selective reflecting filter can be formed of a cholesteric liquid crystal layer. The cholesteric liquid crystal layer selectively reflects a light component in a specific wavelength band corresponding to the helical direction and pitch of liquid crystal molecules, and transmits the remaining light components, as is well known. The cholesteric liquid crystal layer can also be used as a polarization element.
In this case, the cholesteric liquid crystal layer is adjusted so that the selective reflection characteristics cover the entire visible light range. However, in the liquid crystal display device of the present invention, the helical pitch of the cholesteric liquid crystal is set to cover the wavelength band in units of three color pixels, i.e., R, G, and B color pixels. An arbitrary selective reflecting filter is formed to have an average helical pitch different from that of any adjacent selective reflecting filter.
This selective reflecting layer is formed as follows. The entire inner surface of the back-surface-side transparent substrate is coated with a cholesteric liquid crystal, and each selective reflecting filter area is irradiated with an ultraviolet ray while adjusting the irradiation time. Selective reflecting filters having different crosslinking pitches are arrayed in a predetermined cycle to construct one selective reflecting layer. When the selective reflecting layer is formed on the inner surface of the transparent substrate as described above, disparity can be eliminated to improve the display quality of the liquid crystal display device.
Still another liquid crystal display device according to the present invention comprises:
an optical modulation layer which is sandwiched between a pair of observation-side and back-surface-side transparent substrates opposing each other and has a plurality of liquid crystal pixels arranged in a matrix, the optical modulation layer being adapted to modulate incident light in accordance with an applied voltage; a selective reflecting layer having a plurality of selective reflecting filters arrayed in a predetermined cycle, respectively, on back-surface sides of the liquid crystal pixels to selectively and partially reflect light components having different wavelength bands; and a color filter layer having a plurality of color filters disposed to oppose front-surface sides of the selective reflecting filters and having different transmission wavelength ranges in a visible light range,
wherein each color filter has a wavelength range in which a transmittance in the visible light range is not less than 50% and a wavelength range in which a transmittance in the visible light range is less than 50%, and
each of the selective reflecting filters has a reflectance of 50% to 90% for light in the wavelength range in which a transmittance of a corresponding one of the color filters opposing each selective reflecting filter is not less than 50%, and has a reflectance of more than 90% for light in the wavelength range in which a transmittance of the corresponding color filter is less than 50%.
According to the flat display device having the above arrangement, each of the selective reflecting filters has a reflectance of 50% to 90% for light in the wavelength range in which a transmittance of a corresponding one of the color filters opposing each selective reflecting filter is 50% or more. When this display device functions as the reflection type, sufficient reflection brightness can be obtained. On the other hand, even when the display device functions as the transmission type by arranging a back-surface light source having a reflecting plate, a light amount obtained by subtracting the reflectance of each selective reflecting filter from 100% and a light amount reflected by the selective reflecting filter by an amount corresponding to the reflectance and reused via the reflecting plate of the surface light source contribute to the display brightness. That is, the sum of these two light amounts allows the polarization transmittance of the selective reflecting filter to fall within the range of 50% to 90% in the wavelength band. Therefore, display having sufficient reflection brightness can be achieved.
On the other hand, each selective reflecting filter opposing a color filter whose transmittance in the visible light range is less than 50% has a large reflectance of more than 90% for the light in the wavelength range in the visible light range of the color filter. When the flat display device functions as the reflection type, external light passes through the color filter twice to display an image at a sufficiently high saturation. When the flat display device functions as the transmission type, the selective reflecting filter sufficiently reflects light in the above wavelength range toward the back-surface light source side. The light does not pass through the opposing pixel, and therefore a sufficiently high saturation can be obtained in the display.
When the color filter layer is formed on the inner surface of the observation-side transparent substrate, no disparity is produced in colors, and a sufficiently high saturation can be obtained. When the selective reflecting layer is formed on the inner surface of the back-surface-side transparent substrate, disparity due to erroneous perception of the selective reflecting layer as a display shadow can be eliminated.
In the flat display device of the,present invention, the selective reflection wavelength band of each selective reflecting filter is determined to match the color pixel opposing the selective reflecting filter. Such selective reflecting filters are made of cholesteric liquid crystal thin films, and the helical pitches of the cholesteric liquid crystal layers are set to have wavelength bands and polarization reflectances corresponding to the pixels of the respective colors, R (red), G (green), and B (blue). An arbitrary selective reflecting filter is formed to have the helical pitch, number of layers, and film thickness different from any adjacent selective reflecting filter.
The selective reflecting layer can be formed as a single layer having selective reflecting filters having different crosslinking pitches by applying a cholesteric liquid crystal to the inner surface of the back-surface-side transparent substrate and irradiating the selective reflecting filter regions with ultraviolet rays upon adjusting their irradiation times. The selective reflecting layer having different polarization reflectances depending on the wavelength ranges can be obtained as a multilayered structure having different helical pitches of the cholesteric liquid crystal thin films in their direction of thickness. The multilayered structure can be obtained as a multilayered structure in which the adjacent selective reflecting filters have different helical pitches depending on the spectral characteristics of the corresponding color filters, or a multilayered structure having various film thicknesses.
When a selective reflecting filter is designed such that the first wavelength range, i.e., the wavelength range in which the transmittance of the opposing color filer is 50% or more partially overlaps the first wavelength range of any adjacent selective reflecting filter, the dimensional tolerance can have a margin in forming the pattern of the selective reflecting layer.
When the square value of the Y value (Y in tristimulus values) of the average transmittance of the color filter layer is set to 40% or more, a flat display panel capable of displaying an image with sufficiently high brightness at a sufficiently high saturation can be obtained as in the conventional reflection type flat display device.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.